FAQ - What is allow-ttl-without-nsup used for?

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FAQ - What is allow-ttl-without-nsup used for?

Detail

The NSUP or namespace supervisor sub-system is an internal process within Aerospike that takes care of removing records when their time to live has elapsed, a process called expiration or, when certain thresholds are met, before the TTL has elapsed, a process called eviction. By default the time interval between which NSUP scans the primary index for records eligible for expiration is 0, meaning that expiration and eviction are switched off. Unless this nsup-period is configured to a non-zero value, Aerospike will not allow records with a TTL to be written to the database.

The reason for this is that the developers would be expecting records with TTL to be removed via expiration and as such may not have included explicit methods for record removal in their code (such as expunges or durable deletes). If Aerospike allowed records with non-zero TTL to be written when NSUP is not running it could lead to the database being overloaded with records and entering stop writes.

Why then is there a configuration parameter called `allow-ttl-without-nsup’?

Answer

The reason to include allow-ttl-without-nsup is to allow the customer to measure the impact of NSUP when running in a use case where TTL is non-zero. When NSUP runs it locks to a single CPU (by default, but configurable through the nsup-threads configuration parameter) and will utilise that CPU as much as 100% which can be alarming for the operations team. On modern multi core CPU systems, the NSUP process (running in user space) would typically not impact the overall performance. Nevertheless, it can be useful to see exactly what impact NSUP really has on the system.

The way in which this test is carried out is to have a system where nsup-period is set to a non-zero value and records with a TTL are being written. The CPU in normal conditions is measured and then allow-ttl-without-nsup is set to true and nsup-period to 0. The CPU usage and system impact is measured again. The delta between the measurements is the effect of NSUP. The parameter allows the test to be carried out without disrupting incoming client workload on a production system. Once the tests are finished allow-ttl-without-nsup is set back to false and nsup-period is switched back to the usual value for the system. Expiration will then continue as normal.

Notes

Keywords

ALLOW-TTL-WITHOUT-NSUP NSUP EXPIRATION EVICTION CPU 100%

Timestamp

February 2021